119

6 Solventless Chlorination of Glycerol to

Dichloropropanols

120

6.1 INTRODUCTION

Biodiesel production and availability of glycerol as the co-product in ~10% by weight

have been much talked about subjects. Huge quantities of bioglycerol will be

available in future if the correct projections materialize and thus bioglycerol needs to

be valorized to arrive at economically favorable cost of biodiesel (Stelmachowski,

2011; Fan et al., 2010; Yadav et al., 2012a,b,c,d). Although glycerol itself has wide

applications in foods, paint, pharmaceutical, cosmetics, soap, toothpaste industries,

the amount of glycerol produced will be massive and hence its new application per se

or value added derivatives must be produced. Some of them include conversion of

glycerol to acrolein, citric acid, lactic acid, dihydroxyacetone, 1,3-propanediol, 1,3-

and 1,2-dichloropropanols, epichlorohydrin, and a host of ethers.

Dichloropropanols are valuable chemicals which can be produced from glycerol.

They are used as intermediates in epichlorohydrin production. Epichlorohydrin is

enormously versatile intermediate used in wide variety of applications.

Approximately 76% of the world’s consumption of epichlorohydrin is used to make

epoxy resins (Bruin, 1959; Brojer et al., 1965). It has also been used to cure

propylene-base rubbers, as a solvent for cellulose esters and ethers, surface active

agent used in cosmetics and shampoos, as a stabilizer in rubber exhibiting resistance

to extreme temperatures applications in automotive and aircraft parts, seals gaskets,

and agricultural products such as insecticides, bactericides and fungicides and in

resins with high wet-strength for the paper industry (Weissermel and Arpe, 1997).

Epichlorohydrin is prepared industrially by various processes which include high

temperature chlorination of propylene to allyl chloride, followed by hypo-chlorination

of allyl chloride to give dichloropropanols and then by dehydrochlorination with

caustic (Scheme 6.1, Route 1). However, this process has considerable drawbacks

which include sacrificial use of chlorine and complications due to the industrial use

and generation of hypochlorous acid and formation of unwanted chlorinated

hydrocarbons including trichloropropane, chlorinated ethers and oligomers (Scheme

6.2) (Weissermel and Arpe, 1997; Green et al., 2000; Aoki et al., 2011; Nagato et al.,

1987; Kasai et l., 1992; Stephen and Maria, 1991; Kaisha, 1969; Kawabe et al., 1978;

Grimsby, 1986; Grimsby, 1987; Mass and Petrus, 1994; Dettloff and Null, 2005).

Another known process involves insertion of oxygen in allylic position of allyl

chloride using hydrogen peroxide over titano-silicate catalyst (Scheme 6.1, Route 2)

121

(Clerici and Ingallina; 1993; Bhaumik et al., 1994; Gao et al., 1996; Thomas et al.,

1992; Dartt and Davis, 1994). Propylene is converted to allyl alcohol by oxidative

acetoxylation to allyl acetate followed by hydrolysis in the Showa Denko process

commercialized in 1980s. Allyl alcohol is then chlorinated in aqueous HCl to give

glycerol dichlorohydrin, followed by dehydrochlorination using a base to give

epichlorohydrin (Scheme 6.1, Route 3) (Aoki, 1999). Dow’s patented process starts

with propylene oxidation to acrolein in first step, followed by chlorination to 2, 3-

dichloropropanal in second step, which is further hydrogenated to give 2, 3-

dichloropropanol (Scheme 6.1 Route 4). However, in this case 2,3-dichloropropanol

(2,3-DCP) is predominant which is 10 fold less reactive than 1,3- dichloropropanol

(1,3-DCP) in the process of preparing epichlorohydrin (Kenneth et al., 1958). Both

1,3- and 1,2- DCP’s are the starting materials for epichlorohydrin manufacture among

which 1,3-dichloropropanol is almost one order of magnitude more reactive to form

epichlrohydrin. Thus, selectivity to 1,3-DCP is of paramount importance for

commercial operation to reduce reactor size and time. According to an Asahi patent,

acetone produced in the Hock process for phenol can be chlorinated to give

dichloroacetone. Upon hydrogenation of dichloroacetone, as per a related Mitsubishi

patent, dichloropropanol is formed (Scheme 6.1, Route 5). Dehydrochlorination with

a base affords epichlorohydrin (Denttloff and Null, 2005). All the processes for

epichlorohydrin production go through dichloropropanols as intermediates. Glycerol

can be converted to dichloropropanol by reacting it with hydrochloric acid in presence

of organic carboxylic acids as the catalyst followed by dehydrochlorination with a

base to give epichlorohydrin; it offers many economical and environmental

advantages over conventional method (Gosselin et al., 2005; Schreck et al., 2006;

Gilbeau, 2006; Siano et al., 2006; Krafft et al., 2007; Krafft and Gilbeau, 2008; Mehta

et al., 2010).

Substantial work has been done for the conversion of glycerol to dichloropropanols.

All patents describe use of homogeneous organic acids such as carboxylic acid and its

derivatives, dicarboxylic or polycaboxylic acid with 2-10 carbon atoms, an anhydride,

an acid chloride, an ester, a lactone, a lactam, an amide, a metal organic compound

and a metal salt (Gosselin et al., 2005; Schreck et al., 2006; Gilbeau, 2006; Siano et

al., 2006; Krafft et al., 2007; Krafft and Gilbeau, 2008; Mehta et al., 2010). Processes

using homogeneous organic acids as catalysts suffer from considerable drawbacks

122

such as, difficulties in separation of product 1, 3 -dichlorohydrin from corrosive

reaction mixture containing water, organic acid, dissolved HCl; loss of the catalyst

during the reaction due to the low boiling organic acid catalyst such as acetic acid;

and decrease in the rate of reaction due to introduction of water in the reaction

mixture by means of aqueous hydrochloric acid use, which prolongs reaction time.

Scheme 6.1: Various industrial routs to synthesize epichlorohydrin

Scheme 6.2: Side reaction of scheme 4.1 route 1

Lee et al. solved this problem by using heteropolyacids s catalyst (Lee et al., 2008)

but their yields are very low and solubility in reaction mixture leads to separation

problems. Hence it is needed to develop an efficient and reusable heterogeneous

catalytic system.

123

In present study, chlorination of glycerol was studied with hydrogen chloride gas by

using MUICaT-5, which is modified version of zirconia catalyst. The catalyst showed

excellent activity, selectivity and stability for dehydration of glycerol to acrolein as

well as in chlorination of glycerol to dichloropropanol (Yadav et al., 2010a,b; Yadav

and Surve, 2011; Tesser et al., 2007). The reaction parameters were optimized for

chlorination reaction in semi-batch process at atmospheric pressure with continuous

flow of HCl gas. Reaction was also studied in semi-batch process at super-

atmospheric pressure in autoclave with HCl gas pressure and kinetic model has been

developed for the same. These results are novel.

6.2 EXPERIMENTAL

6.2.1 Chemicals and catalysis

All chemical were procured from reputed firm and used without further purification:

Glycerol (LR), (S.D. Fine Chem., Mumbai, India), 3-chloro-1,2-propanediol (Sigma-

Aldrich,USA) hexadecyl amine (M/s. Spectrochem Ltd., Mumbai, India). Hydrogen

chloride gas (M/s. A.A. Traders, Mumbai)

6.2.2 Catalyst Synthesis

MUICaT-5 prepared by method reported by Yadav et al. (Yadav et al., 2010a,b).

6.2.2.1 Synthesis of Hexagonal mesoporous silica (HMS)

5 g Dodecyl amine was dissolved in 41.8 g of ethanol and 29.6 g of distilled water.

20.8 g of tetraethyl orthosilicate was added under vigorous stirring. The addition of

ethanol improved the solubility of the template. The reaction mixture was aged for 18

h at 30 °C. The clear liquid above the white colored precipitate was decanted and the

precipitate HMS was dried on a glass plate. The template was removed by calcining

the resulting material at 650 °C in air for 3 h.

6.2.2.2 Synthesis of MUICaT-5 Catalyst

Zirconium oxychloride (2.39 g) and aluminum nitrate (0.11 g) were dissolved in

distilled water. This solution was then added by incipient wetness technique to 5.0 g

of precalcined HMS. The resulting solid material is then dried in an oven at 110 °C

for 3 hours. The dried material was hydrolysed by ammonia gas and washed with

deionised water until a neutral filtrate is obtained, then it is filtered, dried in an oven

124

for 24 hours at 110 °C. The generation of super acidic centers in to this material is

made by grinding tungustic acid (0.1078 g) with 0.9 g of zirconium hydroxide and

aluminum hydroxide on HMS, followed by hydrothermal treatment and then it is

calcined at 750 °C for 3 hours.

6.2.3 Experimental Setup

6.2.3.1 Semi-batch process at atmospheric pressure

Semi-batch experiments at atmospheric pressure were studied in a 5 cm internal

diameter, fully baffled mechanically agitated glass reactor 100 cm3 total capacity with

reactive distillation assembly. The reactor was kept in an isothermal oil bath whose

temperature could be maintained at a desired value. It was equipped with a 6 bladed-

turbine impeller and equi-spaced baffles. Gaseous hydrogen chloride was fed directly

to the bottom of the reactor through classical dispersing device at a constant flow rate.

6.2.3.2 Semi-batch process at super atmospheric pressure

Semi-batch experiments were also carried out at super atmospheric pressure in a Parr

Autoclave made of non corrosive Hastalloy-C with 100 cm3 capacity which was with

magnetically driven. It consisted of a four-pitched blade turbine impeller, temperature

indicator controller, speed controller, pressure controller and thermocouple. Gaseous

hydrogen chloride from a cylinder was fed directly to the bottom of the reactor and its

flow maintained with a flow controller at a desired pressure.

6.2.4 Reaction procedure

6.2.4.1 Semi-batch process at atmospheric pressure

In a typical experiment, 40 g (0.43 mol) of glycerol was fed to the reactor. The reactor

was kept in an isothermal oil bath adjusted to the desired temperature and 2 g of

catalyst (5% w/w) was then added. Initial sample was withdrawn and anhydrous

hydrogen chloride gas was passed through the bottom of the reactor at a desired flow

rate. Speed of agitation was kept at 1100 rpm. Further samples were drawn at periodic

intervals up to 10 h.

6.2.4.2 Semi-batch process at super atmospheric pressure

Glycerol 40 g (0.43 mol) was charged along with the catalyst 2 g (5% w/w) in the

reactor. The reactor was heated up to the desired temperature and anhydrous hydrogen

125

chloride gas fed to reactor at the desired pressure. Initial sample was withdrawn and

speed of agitation was kept at 1100 rpm. Further samples were drawn at periodic

intervals up to 4 h. In this process, chlorination proceeds through formation of α-

monohloropropanediol (α-MCPD) and subsequently 1,3- and 1,2-dichloropropanols

are formed. Therefore, chlorination of α-monohloropropanediol (MCPD) was also

studied independently to establish the kinetics of both mono and dichlorination of

glycerol. In a typical experiment α-monochloropropanediol 40 g (0.36 mol) charged

along with the catalyst 1.2 g (3% w/w) in the reactor. The reactor was heated up to the

desired temperature and anhydrous hydrogen chloride gas was fed to the reactor at a

desired pressure. Initial sample was withdrawn and speed of agitation was kept at

1100 rpm. Further samples were drawn at periodic intervals up to 4 h.

6.2.5 Method of analysis

Samples were centrifuged to remove traces the catalyst and weighed quantity reaction

mixture (containing dissolved HCl gas) was dissolved in methanol and neutralized by

passing through sodium carbonate bed. External standard method was used to

calculate conversion and yield; n-hexanol was used as the external standard.

Standardized samples were analyzed by using gas chromatography (Chemito, GC

1000 using BP-20 capillary column (0.25 i.d., 30 m length) and FID. Reaction

products were confirmed by GC-MS (Perkin-Elmer Clarus 500 Model,) with BP-20

capillary column (0.25 i.d., 30 m length).

6.3 RESULT AND DISCUSSION

Scheme 4.3 depicts the reaction pathways in chlorination of glycerol. Both α- and β-

monochloropropanediols (MCPD) are formed.

6.3.1 Semi-batch process at atmospheric pressure

6.3.1.1 Proof of absence of external mass transfer resistance

The effect of speed of agitation was studied in the range of 800–1200 rpm at a catalyst

loading 5 wt% of glycerol under otherwise similar conditions. The glycerol

conversion and dichlorohydrin selectivity were practically the same at these speeds

(Figure 6.1), which indicated the elimination of mass transfer resistance in studied

ranges of speed.

126

Figure 6.1: Effect of speed of agitation

Glycerol, 40 g (0.43 mol), HCl gas flow rate, 40 ml/min, reaction temperature, 160

°C, catalyst loading, 0.06 g/cc, - Glycerol conc. at 900 rpm, 1,3-DCP conc. at

900 rpm, -Glycerol conc. at 1000 rpm, 1,3-DCP conc. at 1000 rpm, -

Glycerol conc. at 1200 rpm, 1,3-DCP conc. at 1200 rpm

6.3.1.2 Effect of HCl gas flow

Effect of anhydrous HCl gas flow was studied in the range of 20-50 cm3/min on yield

of dichloropropanols under otherwise similar conditions (Figure 6.2). As HCl gas

flow was increased, the selectivity towards dichloropropanol increased but this effect

was observed up to 40 cm3/min suggesting mass transfer limitation or deficiency of

HCl in the reaction mass. Since mass transfer resistance was absent, it was the

deficiency of HCl. Further increase in flow rate did not affect selectivity of

dichloropropanols and led to formation of other dehydration products (Figure 6.3). As

the reaction is reversible in nature, presence of water in reaction mixture decreases

rate of reaction. Increase in HCl gas flow rate strips out water formed in the reaction

0

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60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Con

cent

ratio

n (%

)

Time (h)

127

which further increases reaction rate and subsequently selectivity of

dichloropropanols.

Figure 6.2: Effect of HCl gas flow

Glycerol, 40 g (0.43 mol), reaction temperature, 160 °C, catalyst loading, 0.06 g/cc,

speed of agitation, 1000 rpm, Glycerol conc. at 30ml/min, 1,3-DCP conc. at

30 ml/min, Glycerol conc. at 40 ml/min, 1,3-DCP conc. at 40 ml/min,

Glycerol conc. at 50 ml/min, 1,3-DCP conc. at 50 ml/min

0

10

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60

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80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12

Con

cent

ratio

n( %

)

Time (h)

128

Figure 6.3: Effect of HCl gas flow on selectivity of DCP; Reaction time- 10 h

Glycerol, 1,3-DCP, MCPD, other

6.3.1.3 Effect of catalyst loading

Catalyst loading at different catalyst concentration varied over a range of 0.03 to 0.1

g/cc with respect to the total reaction volume at 160 °C. In the absence of external

mass transfer resistance, the rate of reaction is directly proportional to the catalyst

loading based on the entire liquid-phase volume. Percentage Concentrations of

glycerol and dichloropropanol versus time are plotted, which show effect of catalyst

loading on concentration of glycerol and dichloropropanol (Figure 6.4). The rate of

formation of monochloropropanediol from glycerol is very fast and hence there is no

significant effect of catalyst loading on this step-1 (Scheme 6.4). However, the rate of

formation of dichloropropanol from α-monochloropropanediol (Scheme 6.5) is very

slow hence increasing catalyst loading, increases concentration of dichloropropanol.

This effect is consistent up to 0.06 g/cm3 of catalyst loading; the loading above this

shows decrease in selectivity due to formation of other byproducts.

0

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30

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60

70

80

90

100

20 30 40 50

Con

vers

ion/

sele

ctiv

ity (

%)

HCl gas flow(ml/min)

129

Figure 6.4: Effect of catalyst loading

Glycerol, 40 g (0.43 mol), reaction temperature, 160 °C, HCl gas flow rate, 40

ml/min, speed of agitation, 1000 rpm, Glycerol conc. at 0.02 g/cc, 1,3-DCP

conc. at 0.02 g/cc, Glycerol conc. at 0.06 g/cc, 1,3-DCP conc. at 0.06 g/cc,

Glycerol conc. at 0.1 g/cc, 1,3-DCP conc. at 0.1 g/cc

6.3.1.4 Effect of temperature

Effect of temperature was carried over MUICaT-5, with 2 wt% of catalyst

concentration, over a range of 140-160 ºC, under otherwise similar conditions, in the

absence of external mass transfer resistance. The initial rate of reaction is directly

proportional to the temperature. Percentage concentration of glycerol and

dichloropropanol versus time are plotted (Figure 6.5). The rate of formation of

monochloropropanediol from glycerol is very fast, hence there is no significant effect

of temperature on this step-1. However, the rate of formation of dichloropropanol

from monochloropropanediol (Scheme 6.5) is very slow hence increasing temperature

leads to an increase conversion to dichloropropanol. This effect is consistent up to 160

ºC; the temperature above this shows decrease in selectivity due to formation of other

byproducts such as trichloropropanol.

0

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30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12

Con

cent

ratio

n(%

)

Time (h)

130

Figure 6.5: Effect of temperature

Glycerol, 40 g (0.43 mol), HCl gas flow rate, 40 ml/min, speed of agitation, 1000

rpm, catalyst loading, 0.06 g/cc, Glycerol conc. at 140 ºC, 1,3-DCP conc. at

140 ºC, Glycerol conc. at 150ºC, 1,3-DCP conc. at 150 ºC, Glycerol

conc. at 160 ºC, 1,3-DCP conc. at 160 ºC

6.3.1.5 Concentration profile

The concentrations profile at optimized reaction conditions are shown in Figure 6.6. It

shows complete conversion of glycerol to monochloropropanediol in 6 h. Rate of

formation of dichloropropanol from monochlorpropanediol is very slow and, hence it

is the rate determining step.

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0 1 2 3 4 5 6 7 8 9 10 11 12

Con

cent

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n (%

)

Time (h)

131

Figure 6.6: Concentration profile for batch process at atmospheric pressure

Glycerol, α-MCPD, β-MCPD, 1,3-DCP, 1,2-DCP,

Others

6.3.2 Semi-batch process at super atmospheric pressure

As described above rate of formation of monochloropropanediol from glycerol

(Scheme 6.4) is very fast, hence there is no significant effect of any parameter.

However, the rate of formation of dichloropropanol from monochloropropanediol

(Scheme 6.5) is very slow and hence it is the rate determining step. The semi-batch

process at super atmospheric pressure was studied for establishing the kinetics of

chlorination of monochloropropanediol to dichloropropanol.

6.3.2.1 Proof of absence of external mass transfer resistance

The effect of speed of agitation was studied in the range of 800–1200 rpm at a catalyst

loading 3 wt % of monochloropropanediol under otherwise similar conditions, which

corresponded to 0.039 g/cm3 of the reaction volume. The monochloropropanediol

conversion and dichlorohydrin selectivity were practically the same at these speeds

0

10

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30

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60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12

Con

cent

ratio

n (%

)

Time (h)

132

(Figure 6.7), which suggests the elimination of mass transfer resistance in the range of

speeds employed.

Figure 6.7: Effect of speed of agitation

α-MCPD, 40 g (0.36 mol), reaction pressure, 7 bar, reaction temperature, 150 ºC,

catalyst loading, 0.039 g/cc, 800 rpm, 1000 rpm, 1200 rpm

6.3.2.2 Effect of HCl gas pressure

Effect of anhydrous HCl gas pressure was studied in the range of 5-atm on yield of

dichloropropanols under otherwise similar conditions (Figure 6.8). As HCl gas

pressure increases, the selectivity towards dichloropropanol increases. However, this

effect was consistent up to 7 atm HCl pressure; further increase in pressure did not

affect selectivity of dichloropropanols and led to the formation of other dehydration

products. Plot of initial rate vs HCl pressure shows linear increase in initial rate of

reaction with increase in HCl pressure. (Figure 6.9) This is due to higher

concentration of HCl dissolved in the reaction medium.

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90

0 20 40 60 80 100 120 140 160 180 200 220 240

Con

vers

ion(

%)

Time (min)

133

Figure 6.8: Effect of HCl gas pressure

α-MCPD, 40 g (0.36 mol), reaction temperature, 150 ºC, catalyst loading, 0.039 g/cc,

speed of agitation, 1000 rpm, 5 bar, 6 bar, 7 bar, 9 bar

Figure 6.9: Plot of initial rate vs HCl gas pressure

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Con

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(%)

Time (min)

y = 1.2353x R² = 0.9912

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-rA

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6.3.2.3 Effect of catalyst loading

The effect of catalyst loading on α-monochloropropanediol conversion was studied

from 0.01 to 0.09 g/cm3 at 160 °C (Figure 6.10). In the absence of external mass

transfer resistance, the rate of reaction is directly proportional to the catalyst loading

based on the entire liquid-phase volume. It was observed that as the concentration of

catalyst was increased up to 0.039g/cm3, the conversion increased, beyond which there

is no substantial increase. This would mean that the number of sites available is more

than that required. The reaction shows 52% conversion in 4 h time at when no catalyst

was used. This would indicate that acidity provided by dissolved HCl also acted as

catalyst. Thus, the reaction consists of both homogeneous and heterogeneous centers

and this is discussed separately.

Figure 6.10: Effect of catalyst loading

α-MCPD; 40 g (0.36 mol), reaction pressure 7 bar, reaction temperature, 150 ºC,

speed of agitation, 1000 rpm, 0 g/cc, 0.013 g/cc, 0.026 g/cc, 0.039 g/cc,

0.09 g/cc.

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Con

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Time (min)

135

6.3.2.4 Effect of temperature

Effect of temperature on α-monochloropropanediol conversion was carried over

MUICaT-5, with 0.039 g/cm3 catalyst concentration at 7 atm HCl pressure, over a

range of 140-160 ºC, under otherwise similar conditions (Figure 6.11). This effect is

consistent up to 150 ºC; the temperature above this shows decrease in selectivity due

to formation of other byproducts.

Figure 6.11: Effect of temperature on monochloropropanediol conversion

α-MCPD, 40 g (0.36 moles), reaction pressure, 7 bar, speed of agitation, 1000 rpm,

catalyst loading, 0.039 g/cc, 130 ºC, 140 ºC, 150 ºC, 160 ºC

6.3.3 Reaction mechanism and concentration profile

Figure 6.12 shows complete concentration profile of all reaction components at super

atmospheric pressure. It shows complete conversion of glycerol to

monochloropropanediol in 3 h. Rate of formation of dichloropropanol from

monochlorpropanediol is very slow and is the, rate determining step.

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0 20 40 60 80 100 120 140 160 180 200 220 240

Con

vers

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(%)

Time(min)

136

Figure 6.12: Concentration profile for batch process at super atmospheric pressure

Glycerol, α-MCPD, β-MCPD, 1,3-DCP, 1,2-DCP

6.3.4 Catalyst reusability

The catalyst was filtered from the reaction mass and refluxed with methanol in order

to remove any adsorbed material. It was dried at 120 ºC before every use. There was

loss of catalyst due to attribution during filtration. The loss in the catalyst quantity in

every cycle, was made up by adding required amount of fresh catalyst. Catalyst

activity remains constant after 3rd reuse.

6.3.5 Reaction mechanism and kinetics

Chlorination of glycerol to dichloropropanol consists of two step reactions (Scheme

6.3). According to observed concentration profile in Figure 6.12, the 1st step i.e.

glycerol to α-MCPD is fast and 2nd step i.e. α-MCPD to 1,3-DCH is slow. It is rate

determining step. In agreement with experimental observations in the 1st step there is

a marginal difference in catalyzed and un-catalyzed reaction rates, because HCl itself

acts as the catalyst to give α-MCPD. Mechanism of un-catalyzed reaction of glycerol

0

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0 50 100 150 200 250

Con

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Time (min)

137

and hydrochloric acid to give α-MCPD and β-MCPD can be demonstrated in (Scheme

6.4 and 6.5). Nucleophillic substitution occurs at less substituted carbon atom of

oxonium intermediate to give chloride ion substitution at α position to give α-MCPD

predominantly. Although β- substitution can occur it is less favorable. According to

previous report, the observed concentrations are very low (Tesser et al., 2007). While

2nd step is also un-catalyzed reaction, there is a significant difference in catalyzed and

un-catalyzed reaction rates as per experimental observation shown in (Figure 6.10).

Since the 2nd step is rate determining step the mechanism of catalyzed and

uncatalyzed reaction can be explained as follows.

Uncatalyzed reaction mechanism can be same as that of 1st step reaction mechanism

of uncatalyzed reaction of glycerol to give α-MCPD (Scheme 6.5). In the mechanism

for catalyzed reaction, α-MCPD and HCl are adsorbed on adjacent catalyst sites.

Proton on catalyst site attacks terminal –OH group followed by dehydration and

formation of oxonium intermediate. Oxonium intermediate is then attacked by Cl- ion

on terminal carbon atom (Scheme 6.6).

In agreement with experimental observation, the amount of β-MCPD formed in 1st

step is very low and remains constant throughout the reaction, which shows that β -

MCPD does not react further to give 1,2-DCH. This can be explained on the basis of

proposed reaction mechanism of 2nd step. According to it, the formation of oxonium

ion is not possible in the absence of vicinal hydroxyl group since the β-position is

already substituted by Cl- ion in β- MCPD. This explanation is valid for catalyzed

reaction mechanism in agreement with the observed experimental data.

138

Scheme 6.3: Reaction mechanism chlorination of glycerol to dichloropropanol

Scheme 6.4: Mechanism of un-catalyzed reaction of glycerol and hydrochloric acid to

give α-MCPD and β-MCPD

139

Scheme 6.5: Mechanism of un-catalyzed reaction of α-MCPD and hydrochloric acid

to give 1,3-DCP and 1,2-DCP

Scheme 6.6: Mechanism of catalyzed reaction of α-MCPD and hydrochloric acid to

give 1,3-DCP and 1,2-DCP

140

6.3.5.1 Development of kinetic model

Glycerol (A) and hydrochloric acid (B) undergo uncatalyzed reaction to give α-

MCPD(C) and β-MCPD (D). α-MCPD (C), β-MCPD(D) and HCl (B) adsorb on the

adjacent catalyst sites(S) and only α-MCPD (C) undergo catalyzed reaction to form

1,3-DCP(E) and 1,2-DCP(F), which then desorb from catalyst sites. The reaction

mechanism is given by Scheme 6.6.

The studies of the effects of the foregoing parameters on conversion and rates of

reactions suggested that the reaction was free from internal diffusion and external mass

transfer resistances and hence intrinsic kinetic equations could be written as follows.

1KA B C D� �1K1 C D1K1 DC D1K1 C1 (1)

.KcC S C S� Kc C SKc C S.Kc (2)

.BKB S B S� BKB B SBKB B S.BKB (3)

2. . .KC S B S E S S� �2K2 E S2K2 E SE SE SE S.E S2K2 E S2 . (4)

3. . .KC S B S F S S� �3K3 F S3K3 F SF SF SF S.F S3K3 F S3 . (5) 1/. EKE S E S�1/ EKE E S1/ EKE SE S1/ EKE (6) 1/. FKF S F S�1/ FKF F S1/ FKF F SF S1/ FKF (7)

Step 4 is rate determining steps

1C D A BC C K C C� (8)

.

2 2 2 '.C S C S CC C S C SC K C C C K C C� � � (9)

.

2 2 2 '.B S BB B S B S B SC K C C C K C C� � � (10)

. 3 . .F S S C S B SC C K C C� (11)

.E S E E SC K C C� (12)

.F S F F SC K C C� (13)

The overall rate of chlorination of α-MCPD (C) is '

2 . . 2 .c C S B S E S Sr k C C k C C� � � (14)

The first term on RHS represents rate of adsorption of α-MCPD and HCl and second

term on RHS represents rate of formation of 1,3-DCP. Eq. 14 can be simplified as. ' ' 2 ' 2

2 2C S Sc B C B E Er k K K C C C k K C C� � �

2 ' ' '2 2Cc S B C B E Er C k K K C C k K C� � � �� � (15)

141

The site balance can be represented as,

. . . . .t C S D S B S E S F S SC C C C C C C� � � � � � (16)

' ' 'C Dt C S D S B B S E E S F F S SC K C C K C C K C C K C C K C C C� � � � � �

' ' '1C Dt S C D B B E E F FC C K C K C K C K C K C� � � � � � �� � (17)

From eq. 17 rate of reaction can be given as, ' ' ' 2

2 2

' ' '1C

C D

B C B E E tc

C D B B E E F F

k K K C C k K C Cr

K C K C K C K C K C

� �� �� � �� � � � � �� �

(18)

For initial rate of reaction

� �' ' 2

2

' ' '1C

initial

C

B C B tc

C D D B B

k K K C C Cr

K C K C K C� �

� � � (19)

As adsorption constants are very low, ' ' 2

2initial Cc B t C Br k K K C C C� � (20)

0 0B CC C

' ' 22initial Cc B t Cr k K K C C� � (21)

As α- MCPD is produced in the reactor by a uncatalyzed reaction, the overall reaction

will have a rate constant which is summation of uncatalyzed reaction rate constant

and catalyzed reaction rate constant, hence the equation will be.

initialc P Cr k C� � (22)

Where Pk (Pseudo reaction rate constant)

. .P uncat catk k k W� �

W : catalyst loading (g/cc)

kuncat : Un-catalyzed reaction rate constant

kcat : Catalyzed reaction rate constant

CP C

initial

dC k Cdt

�� � �� �� �

(23)

(1 )cP C

dX k Xdt

� � (24)

� �ln 1 C PX k t� � � (25)

Thus plots were made in consonance with the equation (25) at different temperature

considering only the initial rate of reaction. The plot shows the straight lines passing

142

through origin (Figure 6.13). The slope of these lines gives the value of kP. A plot of

ln (kP) vs 1/T was made at different temperatures (Figure 6.14). The slope of the plot

gives value of activation energy to be 10.39 kcal/mol.

The plots were also made in consonance with equation (25) at different catalyst

loading considering only initial rate of reaction. The plot shows straight line passing

through origin (Figure 6.15) .The slope of these lines give the value of kP. The plot of

kP vs w (catalyst loading) was obtained to get a straight line which doesn’t pass

through origin (Figure 6.16). It is so because the un-catalyzed reaction is also possible

which evident from plot of (dX/dt) vs w (Figure 6.17). The slope of the line in the plot

of kP vs w gives intrinsic kinetic constant for catalyzed to be 0.5 cm3/gcat s and

intercept on Y axis gives the uncatalyzed reaction rate constant to be 0.004 s-1.

Figure 6.13: Plot of -ln (1-XC) vs time at different temperature

130 ºC, 140 ºC, 150 ºC, 160 ºC

y = 0.0125x R² = 0.9948

y = 0.0175x R² = 0.995

y = 0.0246x R² = 0.9978

y = 0.0293x R² = 0.9913

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 5 10 15 20 25

-ln(1

-XC)

Time (min)

143

Figure 6.14: Plot of ln (kP) vs 1/T

Figure 6.15: Plot of -ln (1-XC) vs time at different catalyst loadings

0.013 g/cc, 0.026 g/cc, 0.039 g/cc

y = -5232.4x + 8.5848 R² = 0.9881

-4.5

-4.3

-4.1

-3.9

-3.7

-3.5

0.0023 0.00235 0.0024 0.00245 0.0025

ln k

1/T

y = 0.0111x R² = 0.9878

y = 0.0176x R² = 0.9983

y = 0.0246x R² = 0.9978

0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20

-ln(1

-XC)

Time (min)

144

Figure 6.16: Plot of ln (kP) vs catalyst loading (w)

Figure 6.17: Plot of (dX/dt) vs catalyst loading (w)

y = 0.5x + 0.0043 R² = 0.998

0

0.005

0.01

0.015

0.02

0.025

0.03

0 0.01 0.02 0.03 0.04

ln K

P

Catalyst Loading (w) (g/cc)

y = 90.769x + 5.58 R² = 0.9847

5

6

7

8

9

10

0 0.01 0.02 0.03 0.04 0.05

Initi

al ra

te (-

dX/d

t) E-

05

Catalyst loading (w) (g/cc)

145

6.4 CONCLUSION

A green process has been developed for the chlorination of glycerol with hydrogen

chloride gas by using MUICaT-5 as the solid acid catalyst. The reaction parameters

such as catalyst loading, speed of agitation, HCl gas flow rate, HCl pressure and

temperature were optimized for the glycerol chlorination reaction in a semi-batch

process at atmospheric pressure with continuous flow of HCl gas and also in semi-

batch process at super-atmospheric pressure in autoclave with HCl gas pressure. A

kinetic model has been developed for semi-batch process at super-atmospheric

pressure. The reaction is intrinsic kinetically controlled and follows pseudo-first order

kinetics. The activation energy was found out to be 10.39 kcal/mol. The glycerol

conversion and 1,3-DCP selectivity were found out to be 100% and 70%, respectively

in semi-batch process at atmospheric pressure in 10 h. In semi-batch process at super-

atmospheric pressure glycerol conversion and 1,3-DCP selectivity found out to be

100% and 72%, respectively in 4 h. The catalyst is heterogeneous and can be easily

separated from reaction mixture by simple filtration and can be reused several times.

Unreacted monochloropropanediols can be recycled back for chlorination to give 1,3-

DCP.